1. Field of the Invention
The present invention relates to a biaxially oriented laminated film, and more particularly to a biaxially oriented laminated thermoplastic film having improved surface properties suitable for the base film of a magnetic recording media.
2. Description of the Prior Art
As a biaxially oriented laminated thermoplastic film aimed to improve the surface properties, a biaxially oriented polyester film containing practically spherical particles of colloidal silica is known and described in, for example, JP-A-SHO 59-171623. In such a biaxially oriented laminated thermoplastic film, protrusions are formed on the surface by the contained silica particles. By such formation of the protrusions, it may be possible to increase the handling ability and the running ability of the film by reducing the friction coefficient of the surface of the film, to improve the adhesive property of the film with a magnetic layer for use of a magnetic recording media, or to improve the adhesive property of the film with an ink for use of a package.
In the biaxially oriented laminated thermoplastic film disclosed in JP-A-SHO 59-171623, however, since the contained silica particles are randomly distributed over the entire area in the thickness direction of the film, increase of the density of the protrusions formed by the contained particles is restricted to some extent, and the height of the protrusions varies greatly. Therefore, improvement of the running ability of the film achieved by the reduction of the friction coefficient of the film surface, increase of the scratch prevention property (hereinafter, referred to as scratch resistance) of the film surface and the chipping resistance of the film surface, and increase of the adhesive property of magnetic layer or ink to the film surface, are limited to some extent.
Accordingly, the inventors of the present invention proposed a biaxially oriented laminated thermoplastic film in U.S. Pat. No. 5,069,962, which is improved in surface properties by laminating a layer of thermoplastic resin A and a layer of thermoplastic resin B and containing particles having a specified size concentratively in the layer of thermoplastic resin A. By this proposal, protrusions having a uniform height can be formed on the film surface efficiently and at a high density, and the running ability, scratch resistance, chipping resistance and adhesive property of magnetic layer or ink of the film can be greatly improved.
The inventors of the present invention have further proceeded the investigation. As a result, it has been found that a biaxially oriented laminated thermoplastic film having a more excellent surface properties can be made by further laminating another layer on the previously proposed biaxially oriented laminated film.